LOL, how could two "emeritus professors" write an article like this without a single mention of the Mayan civilization, which is regarded by many as the earliest use of an explicit zero in a numerical system. Are they not aware of this?
The Mayans may or may not have been first, but due to their isolation their use of zero was unknown to the rest of the world. By the time Europeans and Mayans met, the Hindu-Arabic numeral system was well established and that was the system that became the world's.
Not sure, but maybe because it isn't a positional zero.